1. Field of the Invention
The invention pertains generally to confocal microscopes and to the application of such microscopes to the fabrication of devices, e.g., semiconductor devices.
2. Art Background
During the fabrication of certain devices, such as semiconductor devices, lithographic processes are used to pattern substrates such as silicon wafers or processed silicon wafers which are, for example, wholly or partially covered by metal, silicon dioxide or polycrystalline silicon. That is, a substrate is coated with an energy-sensitive material called a resist. Selected portions of the resist are exposed to a form of energy which either removes the exposed portions to bare portions of the substrate or more typically induces a change in the solubility or reactivity of the exposed portions in relation to a given developing agent or etchant. The more soluble or reactive portions of the resist are removed, thereby patterning the resist, and the bared portions of the substrate are then treated, e.g., are etched, implanted, or metallized, through the patterned resist.
A significant concern associated with the above-described patterning procedure is the need to achieve good linewidth control during pattern transfer from the patterned resist into the substrate. In this regard, to assure good linewidth control, conventional optical microscopes are currently used to measure the linewidths of features in patterned resists and of device features in patterned substrates. Alternatively, these microscopes are used to measure the linewidths of features in so-called knock-off patterns, i.e., patterns which are representative of the desired device patterns, but formed in one or more portions of the substrate which are discarded and thus not incorporated into the resulting device or devices. If the measured linewidths satisfy a desired criterion, then the device or devices being formed in the substrate are completed. If not, then, for example, the patterned resist (which has failed to satisfy the desired criterion) is removed and a new patterned resist is formed, or the patterned substrate (which has failed to satisfy the desired criterion) is discarded.
As is known, the resolution limit of a conventional optical microscope is the diffraction limit of the microscope objective lens, which is .lambda./2 NA, where .lambda. is the wavelength of the light and NA denotes the numerical aperture. Because features in semiconductor devices are now as small as, or even smaller than, 1 micrometer (.mu.m), and thus the sizes of these features are approaching the resolution limit of conventional optical microscopes (operating at visible wavelengths), a need has arisen for linewidth measurement tools having enhanced resolution capabilities.
A microscope which, in effect, exhibits a smaller resolution than that of the conventional optical microscope is the conventional confocal microscope. That is, as depicted in FIG. 1, a conventional confocal microscope 10 typically includes a single objective lens 30 through which the light is passed twice, i.e., light apertured by a pinhole 20 is focused by the objective lens 30 onto a specimen 40, and light reflected from the specimen 40 is focused by the lens 30 onto the pinhole 20, where it is detected. (The aperture 20 is a pinhole, for purposes of the present disclosure, provided the size, e.g., diameter, of the aperture is less than the diffraction limit of the lens 30 at its long conjugate distance, i.e., at the pinhole 20.) As is known, the conventional confocal microscope 10 exhibits a relatively small depth of focus compared to a conventional optical microscope. Consequently, the conventional confocal microscope 10 is capable of achieving a relatively high contrast between the top and bottom of, for example, a step-like feature in the surface of the specimen 40. Because the edge of the step-like feature is thus more accurately determined, the conventional confocal microscope 10 in effect achieves a longitudinal resolution which is smaller than that achievable by a conventional optical microscope. In addition, the conventional confocal microscope achieves a transverse resolution which is smaller than that achievable by a conventional optical microscope by a factor of approximately 1/.sqroot.2.
Conventional confocal microscopes have been proposed for use, and presumably are currently being used, in measuring linewidths during semiconductor device manufacture. However, because it is expected that future semiconductor devices will have features which are even smaller than those of present-day devices, microscopes are being sought which exhibit a smaller depth of focus and, in effect, a smaller resolution, than that exhibited by the conventional confocal microscope.